1. Field of the Invention
This invention relates generally to continuous stream type ink jet printing systems and more particularly to printheads which stimulate the ink in the continuous stream type ink jet printers by thermal energy pulses.
2. Description of Prior Art
Ink jet printing systems are usually divided into two basic types, continuous stream and drop-on-demand. In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through one or more orifices or nozzles. The stream is perturbated, so that it is broken into droplets at a predetermined fixed distance from the nozzles. At the break up point, the droplets are charged in accordance with varying magnitudes of voltages representative of digitized data signals. The charged droplets are propelled through a fixed electrostatic field which adjusts or deflects the trajectory of each droplet in order to direct it to a specific location on a recording medium, such as paper, or to a gutter for collection and recirculation. In drop-on-demand ink jet printing systems, a droplet is expelled from a nozzle directly to the recording medium along a substantially straight trajectory, that is, substantially perpendicular to the recording medium. The droplet expulsion is in response to digital information signals and a droplet is not expelled unless it is to be placed on the recording medium. Except for periodic, concurrent expulsion of droplets from all nozzles into a receptacle to keep the ink menisci in the nozzles from drying, drop-on-demand systems require no ink recovering gutter to collect and recirculate the ink and no charging or deflection electrodes to guide the droplets to specific pixel locations on the recording medium. Thus, drop-on-demand systems are much simpler than the continuous stream type.
Generally, the ink in a continuous stream type ink jet printer is perturbated or stimulated by a piezoelectric device attached to the printhead so that regular pressure variations are imparted to the ink in the printhead manifold. The piezoelectric device is usually driven at a frequency in the range of 100 to 125 kHz. It is also known that the ink perturbations may be accomplished by electrohydrodynamic electrodes positioned at the printhead orifices and, as discussed in the patents below, certain forms of thermal energy pulses. When a continuous regular perturbation is impressed on the ink flowing through the small nozzles, the perturbation grows along the length of the stream. The optimum operating conditions are obtained when .lambda. divided by D is less than seven and greater than three, where D is the nozzle diameter and .lambda. is the perturbation wavelength. This perturbation results in stream breakup which produces discrete droplets at fixed distances from the nozzles. As mentioned above, the most common method of supplying this perturbation has been to generate pressure waves by using a piezoelectric material. Such material generates a plane wave that travels across an acoustically designed ink reservoir to reach a nozzle plate that contains the orifices or nozzles through which the streams of pressurized ink flows.
Some problems associated with the piezoelectric stimulated ink streams or jets are that it is difficult to achieve uniform nozzle drive in an array of jets because of the complex acoustic interactions of the pressure wave with the acoustic ink jet cavity or reservoirs of the droplet generators. However, stream breakoff length must be uniform so that all jets or streams must break off in the droplet charging electrodes which are at fixed distances from the nozzles. Also, fabrication of droplet generators may be expensive because of the cost of high precision machining of the acoustically designed reservoirs and very expensive materials. Such droplet generators tend to be heavy and bulky. In addition, the large fluid or ink inertia and potential for air bubble entrapment in the acoustic reservoir is a troublesome problem that must be addressed by such continuous stream printers during startup and shutdown of the ink streams. Several approaches to the solution of these problems are evident in the prior art as delineated below, but none have entirely solved them.
U.S. Pat. No. 3,731,876 to Showalter discloses method and apparatus for producing mist-like fluid sprays. The fluid to be sprayed is heated to a temperature where the vapor pressure of the fluid exceeds the pressure in the space into which it is to be sprayed, but is less than the opening pressure of the nozzle. When the fluid leaves the nozzle orifice, it boils instantly, making the effective viscosity and surface tension of the fluid in and past the spray orifice very small, whereby the fluid breaks up into extremely small drops.
U.S. Pat. No. 3,878,519 to Eaton discloses the selective application of heat energy to the ink stream emitted under pressure from a nozzle to reduce the surface tension of successive segments of the ink stream before the ink stream would randomly break up into droplets. Both the quantity of energy applied and the duration of the applied energy control the breakup point of the stream at predetermined distances from the nozzle. The source of heat may be high intensity light converted to heat energy by the ink stream or an annular or partially annular resistive heater positioned within the nozzle and at the nozzle orifice outer surface. The intense light energy is focused on the ink stream downstream from the nozzle.
U.S. Pat. No. 4,128,345 to Brady discloses a matrix printer which selectively applies fluid impulses onto a record medium. The printer comprises a sheet transport, a printhead, an ink supply, a valve assembly, and a data input system. The printhead includes an array of tubes connected to the ink supply and to the valve assembly. The valve assembly includes a separate valve for each tube for controlling the supply of ink thereto. In one embodiment, a heater raises the temperature of the ink passing through the tubes enough to effect printing whenever the ink is ejected from the tubes. In another embodiment, a movable pin is mounted at the distal end of each tube confronting the recording medium, so that it is driven into the recording medium when a valve is opened. In a further embodiment, the movable pins are heated enough to effect printing when the pins are driven into contact with the recording medium. The data input system opens and closes the valves in accordance with input signals such that the impulses of the ink applied to the tubes produce ink marks on the recording medium.
British Pat. No. 2,060,499 to Endo et al and assigned to Canon K.K., discloses an ink jet printhead in the typical thermal ink jet configuration modified from the drop-on-demand expulsion of ink droplets by the generation of instantaneous bubble generation and collapse by placing the ink under pressure to cause it to continually squirt from each nozzle in streams of ink. The ink streams are perturbated by the continuous addressing of the resistors in the ink channels near the nozzles by current pulses at predetermined frequencies to cause continuous, vigorous changes of state of the ink. That is, bubbles are continually produced and allowed to collapse at a sufficient frequency to stimulate the ink in each channel and to cause the ink streams emitted therefrom to break up into droplets at predetermined distances from the nozzles whereat voltages are applied to the droplets as they are formed.
Unfortunately, such printhead configuration used in the continuous stream operating mode causes dramatic reduction in heater lifetimes, consumes greater quantity of power when the bubble generation is required to perturbate the ink streams, and causes severe crosstalk between ink channels. By crosstalk, it is meant that the activation of the resistors in one nozzle produces an undesired effect on the droplet stream issuing from adjacent nozzles.
British Pat. No. 2,072,099 to Sugitani and assigned to Canon K.K., discloses an ink jet printhead and method of manufacture wherein grooves which constitute the ink flow paths or channels are formed in a layer of photosensitive composition placed on the surface of a substrate having the heating elements thereon. The channels are formed so that the heating elements are within the channels.
U.S. Pat. No. 4,220,958 to Crowley discloses a continuous stream type ink jet printer wherein the perturbation is accomplished by electrohydrodynamic (EHD) excitation. The EHD exciter is composed of one or more pump electrodes of a length equal to about one-half the droplet spacing. The multiple pump electrode embodiments are spaced at intervals of multiples of about one-half the droplet spacing or wavelength downstream from the nozzles.